Laser Powder Bed Fusion and Liquid Metal Replacement for Thermally Stable Invar-Cu and Invar-Ag Composites in Freeform Reflector Applications 73 views

Freeform reflectors require materials with both low coefficients of thermal expansion (CTE) and high thermal conductivity, yet few monolithic systems satisfy this trade-off. Invar-Cu composites were first investigated, combining Invar’s low CTE with Cu’s high conductivity. Conventional machining suffers from tool wear, and sintering produces non-bicontinuous microstructures, whereas laser powder bed fusion enables near-net-shape fabrication of dense parts with bicontinuous architectures. Using a defocused laser to promote conduction-mode melting, dense Invar-Cu was produced with cellular microstructures of Invar-rich cells and percolating Cu channels that act as heat-dissipation pathways. Post-annealing purified both phases, lowering CTE and improving transport, though time-domain thermoreflectance (TDTR) revealed no anisotropic conduction from cell alignment. The thermal conductivity of Cu channels remained limited by dissolved Fe and Ni. To overcome this, Invar-Ag composites were pursued, leveraging Ag’s immiscibility with Fe and Ni to form purer, high-conductivity networks. Because liquid immiscibility prevents direct solidification, a novel liquid metal replacement strategy was developed to selectively substitute interdendritic Cu with Ag within Invar-Cu. Arc-melted precursors permitted millimeter-scale infiltration, yielding bicontinuous Invar-Ag composites with markedly enhanced thermal transport, while LPBF-derived precursors posed challenges due to capillary instabilities in fine cellular structures. Infiltration mechanisms and microstructure evolution were linked and direct evidence of Cu wetting layer by interfacial segregation at Invar/Ag interface were provided by transmission electron microscopes.

PHD (Doctor of Philosophy)

Laser Powder Bed Fusion; Thermal Properties; Liquid Metal

English

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2025-11-24